In some imaging applications, high contrast is desired for objects being in some specific regions, while the same high contrast is not needed for objects being outside of these regions. For example, in some cases a high contrast is required for “near” and “far” objects, while the same contrast is not needed for intermediately distanced objects, very close objects, and, in some case, very far objects. Such dual-range, or, in the more general case, multi-range, imaging is typically performed with bifocal or, respectfully, multifocal lenses.
In ophthalmic applications, bifocal lenses, dividing incident light into two foci or two diffractive orders, are used to provide both near and distant vision. The diffractive bifocal lenses can be advantageously used for reducing lens thickness, while correcting for presbyopia. Multifocal, in particular trifocal, lenses are often used in eyeglasses. They may be configured for reading, working on computer, and far seeing.